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Abstract

Continuous tuning over a 1.6 THz region in the near-infrared (842.5-848.6 nm) has been achieved with a hybrid ring/external cavity laser having a single, optically-driven grating reflector and gain provided by an injection-seeded semiconductor amplifier. Driven at 532 nm and incorporating a photonic crystal with an azobenzene overlayer, the reflector has a peak reflectivity of ~80% and tunes at the rate of 0.024 nm per mW of incident green power. In a departure from conventional ring or external cavity lasers, the frequency selectivity for this system is provided by the passband of the tunable photonic crystal reflector and line narrowing in a high gain amplifier. Sub - 0.1 nm linewidths and amplifier extraction efficiencies above 97% are observed with the reflector tuned to 842.5 nm.

Figures (12)

Diagram in cross-section (not to scale) of the photonic crystal reflectance filter. The acronyms PET, UVCP, and Azo/IPA denote polyethylene terephthalate, ultraviolet-cured polymer, and the azobenzene-isopropyl alcohol solution, respectively. The index of refraction (n) for each component of the reflector is also indicated.

Superposition of several spectra characteristic of the tunable ring laser: (blue) emission from the semiconductor amplifier (in isolation) for a drive current of 190 mA; (green) reflectance spectrum for the photonic crystal; (red) output of the ring laser. The latter two spectra were recorded when no 532 nm power (cf. Fig. 5) was directed onto the tunable reflector (i.e., Pext = 0).

Comparison of the normalized spectra generated by the free-running SOA (green) and the injection-seeded ring laser (red). In recording the free-running spectrum, the SOA current was fixed at 120 mA whereas, for injection-seeded operation, the current was maintained at 190 mA. The injection-seeded spectrum was recorded for Pext = 0. Also, the inset illustrates the experimental arrangement with which the free-running spectrum was acquired.

Laser spectra recorded for three values of the SOA current (120 mA [red], 160 mA [green], and 180 mA [blue]) when the reflector of Fig. 1 has no superstrate (azobenzene/isopropyl alcohol) layer. The mode spacing in all of the spectra is attributable to the thickness of the reflector’s PET substrate. For the sake of comparison of the three spectra, the 160 mA and 180 mA spectra have been attenuated.

(a) Superposition of six laser output spectra, each recorded with the reflector centered at a different wavelength. The green (532 nm) laser power Pext driving the tunable filter is indicated for each spectrum. All of the data were acquired with the SOA driving current maintained at 190 mA; (b) Same data as those of panel (a) but presented with the ordinate having a logarithmic scale.

Dependence of the laser extraction efficiency on the resonant wavelength of the tunable reflector (the injection wavelength, λinj) and, hence, the laser output. Representative estimated uncertainties (one standard deviation) are indicated for several of the measurements and the curve drawn through the data is intended only as a guide to the eye. Note that zero for the ordinate at left has been suppressed. In all of the experiments conducted to date, the amplifier extraction efficiency exceeded 33% and values above 97% were observed.